This invention relates to an acceleration slip controller for controlling slippage that occurs between driven wheels and a road surface when a vehicle accelerates.
One such prior-art acceleration slip controller is shown in Japan Published Unexamined Patent Application Nos. 61-286542 and 62-7954. These controllers detect slippage that occurs between driven wheels and the road surface when the vehicle accelerates, based on the acceleration rate or the revolution speed of the driven wheels and the vehicle speed. In response to detected slippage, the controllers control the revolution speed of the driven wheels until slippage is eliminated by controlling a brake, or by opening or closing a throttle valve of an engine, thus improving the accelerating ability of the vehicle.
Another such controller controls the revolution speed of the driven wheels using a variable X, e.g., the speed of increasing or decreasing a brake oil pressure, the speed of opening or closing the throttle valve, etc. The variable X is determined based on the deviation .DELTA.V of the actual driven-wheel revolution speed from a target driven-wheel revolution
speed calculated, for example, by using the following formula: EQU X=a.multidot..DELTA.V+b.multidot..DELTA.V (1)
In the formula (1), a and b are factors. The target driven-wheel revolution speed is determined from the vehicle speed. With the differential and proportional control using the variable X, the actual driven-wheel revolution speed can approximate the target driven-wheel revolution speed quickly after the control starts.
The acceleration slip control, using the controlled variable X, can make the actual driven-wheel revolution speed approximate the target driven-wheel revolution speed more quickly, than a control where the detected driven-wheel revolution speed exceeds the target driven-wheel revolution speed and the driven-wheel speed is controlled by using a predetermined controlled value: for example, by increasing or decreasing the brake oil pressure at a predetermined rate, by opening or closing the throttle valve at a predetermined speed, etc. The acceleration slip control using the controlled variable X can control friction between the driven wheels and the road surface, thus improving the acceleration ability of the vehicle. However, if a driven-wheel slip rate leaves the predetermined range after the acceleration slip control starts or after the driven-wheel revolution speed drops during the control, the actual driven-wheel revolution speed slowly approximates the target driven-wheel revolution speed.
Specifically, as shown in FIG. 2, the target driven-wheel revolution speed is determined using the .mu.-S curve (.mu.: a friction factor for tires on a road surface, S: slippage rate) and a lateral drag on the tires, so that the driven-wheel slip rate S is about 5 to 10%. Regions A, B and C in FIG. 2 correspond to ranges A, B and C in FIG. 10. The driven-wheel revolution speed is controlled to stay in range B in FIG. 10. In the corresponding range B in FIG. 2, as the slippage rate S increases, the friction factor .mu. gradually increases. A controlled variable for the acceleration slip control is calculated based on the deviation of the actual driven-wheel speed from the target driven-wheel speed, for example, using the mentioned formula (1). In the formula (1) the factors a and b are predetermined so that the actual driven-wheel speed in the range B can quickly approximate the target driven-wheel speed.
With the acceleration slip control, the driven-wheel speed in the range B can quickly approximate the target driven-wheel speed.
The acceleration slip control starts, when the actual driven-wheel speed reaches or exceeds a control start point which is set greater than the target driven-wheel speed; that is, when the slippage rate S reaches or exceeds a point K in FIG. 2, or when the actual driven-wheel speed exceeds the target driven-wheel speed continuously for a predetermined time. When the slippage rate S reaches the range C in FIG. 2, the acceleration slip control starts. The acceleration slip control is thus prevented from starting when the driven-wheel speed temporarily increases; for example, when the driven wheels run on a bumpy road surface.
In the range C in FIG. 2, as the slippage rate S increases, the friction factor .mu. decreases. When the acceleration slip control starts, the slippage rate S quickly increases. Since the controlled variable is calculated using the formula that is established to efficiently control the driven-wheel speed in the range B, the calculated, controlled variable is insufficient for controlling the driven-wheel speed in the range C. The actual driven-wheel speed in the range C slowly approximates the target driven-wheel speed. As a lateral drag decreases, the stability of the running vehicle decreases.
After the acceleration slip control starts, the driven-wheel speed may drop excessively, then the slippage rate S enters the range A in FIG. 2. In the range A, as the slippage rate S varies, the friction factor .mu. changes greatly. To increase the dropped slippage rate S up to the range B, the driven wheels require much driving force. However, since the controlled variable is determined so that efficient controllability of the slippage rate S in the range B can be obtained, the calculated, controlled variable is insufficient for increasing the slippage rate S from the range A to the range B. The driven-wheel speed in the range A requires time to approximate the target driven-wheel speed. The accelerating ability of the vehicle drops accordingly.